My 2001 tt has a severe whistling noise when under full boost. It literally screams (its actually not an unpleasant noise), but I have never heard another one do that. The car hits .7 boost without fail and seems to run fine. Does anyone know if the noise is normal or something that should concern me? If so, any guesses as to what could be causing it? If it was the diverter valve going bad or a torn hose, wouldn't I be losing boost?

 

It should. If a turbo actually whines and it is not an air leak the turbo is coming apart. Could be the compressor running into the housing. You will want to check the turbos for excessive play.

 

Fortunately, the car is still under warranty, but the turbos were checked out before I bought the car a few months ago (it made the noise then as well) and no one considered such a noise a problem. I also have a 993tt and the 996tt is clearly faster. I am wondering if there is just a small pinhole in one of the hoses that is allowing it to whistle, but small enough not to cause a loss of boost. A turbo coming apart is a major concern. thanks.